Updated at 12:55 p.m. ET: United Nations monitors in Syria have suspended operations because of the increasing violence over the last 10 days by President Bashar al-Assad's forces and rebels seeking his overthrow, the head of the observer mission said on Saturday.

"The observers will not be conducting patrols and will stay in their locations until further notice," General Robert Mood said in a statement.

Mood said the intensification in violence was impeding the 300 unarmed U.N. monitors from carrying out their mandate to observe an April 12 ceasefire deal, which has failed to halt the violence.

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"This suspension will be reviewed on a daily basis. Operations will resume when we see the situation fit for us to carry out our mandated activities," Mood said.

The White House called again on the Syrian regime to uphold its commitments under the Annan Plan, including the full implementation of a ceasefire, NBC News reported.

"At this critical juncture, we are consulting with our international partners regarding next steps toward a Syrian-led political transition ... The sooner this transition takes place, the greater the chance of averting a lengthy and bloody sectarian civil war," the White House said in a prepared statement obtained by NBC News.

Many hundreds of people, including civilians, rebels and government forces, have been killed in the two months since international mediator Kofi Annan's ceasefire deal was supposed to come into effect.

Last week shots were fired at a car carrying U.N. observers after they were turned away from the town of Haffeh by angry Assad supporters who threw stones and metal rods at their convoy, a spokeswoman for the monitors said.

Washington and its Western allies have shown no appetite for a Libya-style military intervention even as Moscow has helped shield Assad from tougher U.N. measures.

Diplomats said Mood is expected to brief the U.N. Security Council on Monday or Tuesday about the unrest in Syria.

President Barack Obama will hold talks with Russian President Vladimir Putin on Monday on the sidelines of a G20 summit in Mexico, but expectations are low for any significant progress for breaking the deadlock on Syria.

Reuters contributed to this report.

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